Flexural plate wave (FPW) devices are gravimetric sensors capable of detecting mass changes as small as 10−11 g. Typically, FPW devices are built with a bulk micro-machining process which produces a thin film membrane of silicon or silicon nitride by etching a cavity through the entire thickness of the silicon wafer with a selective process which does not attack the membrane material. However, due to the crystal structure of the silicon wafer, the cavity produced by this etching process has interior walls which extend through the silicon wafer at an angle of 126° from the membrane. This results in the cavity having an opening at the bottom surface of the substrate which is at least twice as large as the area of the membrane. Accordingly, the smallest possible FPW device built utilizing the prior art bulk micromachining process is approximately 1 mm×1 mm, since, for this one square millimeter of area on the surface of the silicon wafer, at least twice as much area is required on the bottom of the wafer. Therefore, only small numbers of FPW sensors can be integrated onto the same silicon chip for exposure to the same environment. For applications which require several sensors with different coatings, several packaged sensors must be integrated onto a sensor assembly and exposed to a gas or liquid sample stream. This method is only practical for applications requiring less than approximately 20 separate sensors.